1. Field of the Invention
The present invention relates to a two-dimensional contact image sensor that is to be used with a facsimile, a scanner or an optical character reader, etc. More particularly, the present invention relates to a driving device and method for a two-dimensional contact image sensor being capable of reducing the number of driving ICs and also being capable of reading the images accurately.
2. Description of the Conventional Art
A conventional two-dimensional contact image sensor is shown in FIG. 5 which is an equivalent circuit diagram of the sensor.
The sensor portion which is a structural unit of the image sensor that comprises a transparent substrate 1 having formed thereon a light-receiving element (photodiode) 2 which is a photoelectric transducer, a thin-film transistor (TFT) 3 which is a switching element, and a lighting portion. A plurality of such sensor portions are arranged in rows and columns in a two-dimensional matrix to form a sensing area.
The transparent electrode of the light-receiving element 2 in each sensor portion is connected to the drain electrode of the thin-film transistor 3. The gate electrodes of individual thin-film transistors are connected to a common gate line 13 for each row, which gate lines 13 being connected for each row to a shift register 7 which controls the turning on and off of the thin-film transistors for each row. The source electrodes of the thin-film transistors 3 are connected to a common data line 14 for each column, which data lines 14 in turn are connected to analog multiplexers 8 for reading in charges.
In the image sensor having the configuration described above, charges are generated in the light-receiving elements 2 in accordance to the quantity of reflected light from the document surface and, as the thin-film transistors 3 are turned on and off, the charges are transferred sequentially into the analog multiplexers 8 for each row, thence read out to produce output image signals (referring to Unexamined Published Japanese Patent Application No. 62980/1989).
In the conventional two-dimensional contact image sensor described above, the gate lines 13 which control TFTs for each row are provided in one-to-one correspondence with the terminals of the shift registers 7 and, hence, it has been necessary to provide as many terminals on the shift registers 7 as the gate lines 13. In addition, the charges generated in the sensor portions are to be read into the analog multiplexers 8 at a time by means of the operation of the common gate lines 13 and, hence, it also has been necessary to provide as many terminals on the analog multiplexers 8 as the data lines 14.
For example, in the case of a two-dimensional contact image sensor of an M.times.N matrix which consists of M gate lines and N data lines; if the number of terminals on a shift register is m whereas the number of terminals on an analog multiplexer is n, it is necessary to use at least M/n shift registers and at least N/n analog multiplexers.
If M.ltoreq.n and N.ltoreq.n, only one shift register and analog multiplexer need be used to compose a two-dimensional contact image sensor; however, if it is required to enlarge the sensing area in order to increase the reading range as in the case of M=10 m and N=10 n, it has been necessary to use ten each of shift registers and analog multiplexers, leading to a higher production cost.
Another problem with the conventional two-dimensional contact image sensor is that imagelags will be produced in the frame scan direction; this problem is discussed below on the basis of FIGS. 6 and 7 with reference to a method of driving an image sensor composed of photodiodes and thin-film transistors. It should be noted that the "capacitance division" which is described hereunder is just one major cause, and by no means the sole cause, of the occurrence of imagelags. The same explanation as given below will also hold for the case of an image sensor that is composed of photodiodes and blocking diodes.
FIG. 6 is an equivalent circuit diagram for one pixel in the image sensor of the type just described above, and FIG. 7 is a timing chart for photodetection from one pixel.
When light falls on a photodiode (PD) supplied with a reverse bias voltage (V.sub.B), a photocurrent ip is generated and supplied to a photodiode capacity (C.sub.PD), an added capacity (C.sub.ADD) and the overlap capacity (C.sub.GD) of a thin-film transistor (TFT), which capacities are all on the PD side. The charges stored for a predetermined time in the capacities on the PD side will turn on the TFT which is a switching element, whereby those charges are transferred to a signal line capacity (C.sub.L) and the overlap capacity (C.sub.GS) of TFT which are both capacities on the signal line side.
Signals are to be fed to the analog multiplexers with high impedance by voltage detection and, hence, all the charges generated are stored in the capacities in the Circuit. In other words, the charges are to be redistributed between the group of capacities on the PD side and the group of capacities on the signal line side. After detecting the potential (V.sub.L) on the signal line side following the end of charge transfer, resetting of charges is effected with a reset switch in order to transfer the charges generated from the PDs in the next row.
The basic characteristics of charge transfer are described below with reference to Equations (1) to (7): ##EQU1## where Ttrans denotes the transfer time; Tstorage, the storage time; V.sub.P0 and V.sub.L0, the potentials on the PD and signal line sides, respectively, at the time of resetting; np, the quantity of light; V.sub.P, the potential on the PD side; and V.sub.L, the potential on the signal line side. In terms of TFT, V.sub.P and V.sub.L correspond to the drain and source potentials, respectively, and they are related to the photocurrent ids expressed by Eq. (7).
V.sub.PP in Eq. (1) is expressed by Eq. (5) and it denotes the potential on the PD side just before charge transfer. .DELTA.V.sub.P and .DELTA.V.sub.L are expressed by Eqs. (3) and (4), respectively, and denote the potentials on the PD and signal line sides, respectively, which will increase on account of the feedthrough that develops when the gate of TFT turns on. F.sub.1 and F.sub.2 in Eqs. (6) and (7), respectively, are functions that denote V.sub.PP, V.sub.P and V.sub.L which will vary with time and they are determined by PD and TFT characteristics.
Charge transfer is continued until V.sub.P becomes equal to V.sub.L across the TFT. If the quantity of stored charges on the PD side (Q0) and the quantity of transferred charges (.DELTA.Q) are considered while omitting the feedthrough, Equation (8) will be derived from Eqs. (1) and (2) taken in combination with the relationship V.sub.P =V.sub.L. The ratio, .eta., between the quantity of charges transferred to the signal line side and the quantity of stored charges on the PD side is expressed by Equation (9), and the proportion of charges left on the PD side, or the relative residual charges .zeta., is expressed by Equation (10). From the relationship between the photocurrent ip generated in the process of charge storage and the photocurrent ids flowing in the process of charge transfer, the ratio .eta. transient is obtained as expressed by Equation (11). ##EQU2##
The case of conducting plural scan cycle is described below using a simplified model of repeated "photo" and "dark" cycles, in which n scans are performed in an illuminated state ("photo"), followed by m scans in a non-illuminated state ("dark").
Suppose first the case of transition from the "dark" to "photo" state The quantity of charges Q1 which are stored in the first scan cycle is expressed by Q1=Qp+Qi, where Qp is the quantity of charges stored by one scan cycle and Qi is the quantity of residual charges that have been left before that scan is started. The quantity of charges transferred to the signal line side (QP1) as a result of the first scan and the quantity of charges left on the PD side (QR1) are expressed, respectively, by QP1=.eta.Q1=.eta.(QP+Qi) and QR1=.zeta.W1=.zeta.(QP+Qi).
If the same reasoning is applied to the second scan cycle, the quantity of stored charges (Q2), the quantity of transferred charges (QP2) and the quantity of residual charges (QR2) are expressed by the following equations, respectively: ##EQU3##
In a similar manner, Qn, QPn and QRn for the nth scan cycle are respectively expressed by Equations (12), (13) and (14): ##EQU4##
In the case of transition from the "photo" to "dark" state, the quantity of charges that have been stored on the PD side before charge transfer for the mth scan cycle is started is expressed by Equation (15). The quantity of charges transferred to the signal line side (q.sub.Dm) as a result of the mth scan and the quantity of charges left on the PD side (q.sub.Rm) are expressed by Equations (16) and (17), respectively: ##EQU5##
In the case where all scans are performed in the "photo" state, the quantity of charges stored on the PD side (Qp) is calculated by bringing n in Eq. (13) for Qpn to infinity (see Equation (18) below). In the case where all scans are performed in the "dark" state, the quantity of charges transferred to the signal line side (q.sub.D) is calculated by bringing m and n in Eq (16) for q.sub.Dm to infinity and zero, respectively, and, hence, q.sub.D is expressed by Equation (19). ##EQU6##
The ratio of capacitance division is calculated by normalizing with Q.sub.p - q.sub.D the charges transferred to the signal line side in each scan cycle. The ratio of capacitance division for the case of "dark" to "photo" transition (imagelag Pn) is expressed by Equation (20) whereas the ratio of capacitance division for the case of "photo" to "dark" transition (imagelag dm) is expressed by Equation (21). The effects represented by Eqs. (20) and (21) are imagelags that develop in the frame scan direction. EQU imagelag.sub.Pn =1-.zeta..sup.n ( 20) EQU imagelag.sub.dm =.zeta..sup.m (1-.zeta..sup.n) (21)
Thus, the charges generated in light-receiving elements are redistributed by the ratios of capacitance on the side of light-receiving elements to capacitance on the signal line side and the charges remaining on the side of light-receiving elements will produce an imagelag. If the next document is read in the presence of residual charges, the generation of charges in response to the new document in addition to the already existing residual charges has made it impossible to produce correct image signals.
One of the methods that have heretofore been adopted to eliminate the production of imagelags in the frame scan direction due to residual charges comprises providing a reset TFT for each pixel, turning on the reset TFT after every scan so that the charges on the side of light-receiving element are brought to zero and then performing the next scan cycle. An equivalent circuit diagram for a two-dimensional contact image sensor that can be used to implement this approach is shown in FIG. 8.
The conventional two-dimensional contact image sensors have had the problem that in order to increase the sensing area, thereby providing more pixels, a greater number of shift registers and analog multiplexers must be used, leading to a higher production cost.
Furthermore, the conventional two-dimensional contact image sensors which provide reset TFTs for eliminating the residual charges have had the problem that the need for providing a reset TFT in each pixel makes it impossible to insure an adequate area for light reception, leading to a lower sensitivity and resolution, as well as a lower yield in the manufacturing process.
In either type of methods adopted to eliminate the residual charges, transition from the "photo" to "dark" state is achieved by resetting charges and, hence, in the subsequent reading of the document in the "photo" state, the ratio of capacitance division dictates that charges are transferred to the capacities on the signal line side in a smaller amount than the charges actually stored in the photodiode, thereby lowering the dynamic range of the image sensor.